Abstract: Affected by PMD(Polarization Mode Dispersion), the polarization state of the single photon will be randomly and slowly altered during the transmission in the fiber. This increases the error probability at the receiving end. So a means for measuring the variance to the preprocess polarization state is required for the distribution of the quantum key. A sensor with balanced detection is introduced to the BB84 protocol and the theoretical model of the QBER (Quantum Bit Error Rate) is established with POVM (Positive Operator Valued Measurement). The variance of the polarization state is measured with the Wollaston prism and the error is eliminated for the dual channel. The angle tolerance of the Wollaston prism is 0.1° and the numerical analysis by the use of the QBER model shows the new protocol decreases the error rate to 3.05×10^-6.

Key words: quantum communications, quantum key distribution, polarization mode dispersion, Wollaston prism, BB84